Weakly-Coupled Few-Mode Gain-Flattening Filter Using Long-Period Fiber Grating in Double-Cladding FMF

A weakly-coupled few-mode gain-flattening filter (FM-GFF) based on long-period fiber gratings (LPFGs) in double-cladding few-mode fiber is proposed. Utilizing the FM-GFF, we demonstrate that the gain spectra of each core mode can be independently flattened.